<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="/static/jquery-1.11.3.js"></script>

    <script>
        $(function () {
           $('#btn').click(function (){
               $('#sel').val(3);
           })
        });
    </script>
</head>
<body>

<select id="sel">
    <option value="1">选项1</option>
    <option value="2">选项2</option>
    <option value="3">选项3</option>
    <option value="4">选项4</option>
</select><br>
<button id="btn">回显第3个选项</button>

</body>
</html>